In view of the numerous studies showing the production and efficient repair of cyclobutyl dimers in normal cells, it has become vital to evaluate the production and repairability of other UV-induced products, which, if inefficiently repaired, could play a large or even major role in cellular UV sensitivity. Progress has been made in a number of areas. Photoproducts involving purines as precursors appear to be produced by UV irradiation of DNAs. These new compounds are formed simultaneously with pyrimidine dimers and adducts. Enzymatic hydrolysis under mild conditions is being used to isolate these products so that sufficient quantities may be accumulated for actual identification. Subsequent to the isolation and characterization of homodimers of methylcytosine derivatives, a similar study was accomplished with cytosine, cytidine, and deoxycytidine. 06,5'-cyclo-5,6-dihydro-2' -deoxyuridine diastereosomers novel anhydro-cyclic nucleosides formed concommitantly with photohydrate of deoxyuridine in water by UV irradiation of deoxyuridine, have been sucessfully separated, isolated, and characterized. Bromination of cytosine derivatives in water resulted in the solution of the elusive bromohydrin derivatives of cytosines. The elucidation of the reaction mechanism was made possible by the study of the formation and reactivities of these bromohydrins.